Simple answer. Would you like to have some slf
come bimbling down the aisle with his scanner in one hand asking "Why did you read the heading back wrong..?" or "Why are we number 63 in the sequence..?" or "I don't want to divert to Belfast..." or the thousand and one other possibilities that would suddenly materialise if you allowed them the power to hear ATC in the cabin.
I would love to be able to lug-in whilst en-route, but as far as the flight-deck goes, ignorance [for pax] is bliss.